1. Field of the Invention
This invention relates generally to drilling oil wells. More specifically, the invention relates to directional drilling and the use of downhole steering. Even more specifically, the invention relates to an apparatus for transferring power between a rotating member and a non-rotating member of a bottom hole assembly.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes or wellbores are drilled by rotating a drill bit attached to the bottom of a drilling assembly (also referred to herein as a xe2x80x9cBottom Hole Assemblyxe2x80x9d or xe2x80x9cBHAxe2x80x9d). The drilling assembly is attached to the bottom of a drill tube, which is usually either a jointed rigid pipe (commonly referred to as the drill pipe) or a relatively flexible spoolable tubing (commonly referred to in the art as the xe2x80x9ccoiled tubingxe2x80x9d). The string comprising the tubing and the drilling assembly is usually referred to as the xe2x80x9cdrill string.xe2x80x9d When jointed pipe is utilized as the tubing, the drill bit is rotated by rotating the jointed pipe from the surface and/or by a mud motor contained in the drilling assembly. In the case of a coiled tubing, the drill bit is rotated by the mud motor. During drilling, a drilling fluid (also referred to as the xe2x80x9cmudxe2x80x9d)is supplied under pressure into the tubing. The drilling fluid passes through the drilling assembly and then discharges at the drill bit bottom. The drilling fluid provides lubrication to the drill bit and carries to the surface rock pieces disintegrated by the drill bit in drilling the borehole. The drilling fluid passing through the drilling assembly rotates the mud motor. A drive shaft connected to the motor and the drill bit rotates the drill bit.
It is well known that formations capable of producing significant amounts of oil and gas (hydrocarbons) are increasingly difficult to find. In addition, economic, political and environmental concerns can make it impossible to place a drilling system directly over a promising formation. As a result, a substantial proportion of the current drilling activity involves drilling of deviated and horizontal borehholes to more fully exploit the hydrocarbon reservoirs. In deviated and horizontal drilling, the wellbore is intentionally drilled at an angle from vertical by special downhole drilling tools to guide the drill assembly in the desired direction. These wellbores are drilled to reach a part of a formation or reservoir, which cannot be drilled by a straight or vertical hole because of the environmental, political, or economic reasons mentioned. Such boreholes can have relatively complex well profiles. To drill such complex boreholes, steerable drilling assemblies are sometimes utilized. A particular drilling assembly includes a plurality of independently operable force application members to apply force on the wellbore wall during drilling of the wellbore to maintain the drill bit along a prescribed path and to alter the drilling direction. Such force application members may be disposed on the outer periphery of the drilling assembly body or on a non-rotating sleeve disposed around a rotating drive shaft. These force application members are moved radially outward from the drilling assembly by electrical devices or electro-hydraulic devices to apply force on the wellbore in order to guide the drill bit and/or to change the drilling direction outward. In such drilling assemblies, there exists a gap between the rotating and the non-rotating sections. To reduce the overall size of the drilling assembly and to provide more power to the ribs, it is desirable to locate the devices (such as motor and pump) required to operate the force application members in the non-rotating section. It is also desirable to locate electronic circuits and certain sensors in the non-rotating section. Thus, power must be transferred between the rotating section and the non-rotating section to operate mechanical devices and the sensors in the non-rotating section.
In drilling assemblies which do not include a non-rotating sleeve as described above, it is desirable to transfer electrical and mechanical power between the rotating drill shaft and the stationary housing surrounding the drill shaft. The power transferred to the rotating shaft may be utilized to operate sensors or mechanical devices in the rotating shaft and/or drill bit. Power transfer between rotating and non-rotating sections having a gap therebetween can also be useful in other downhole tool configurations.
The present invention, which is especially desirable in a space-restrictive application such as the drilling of very small deviated boreholes, provides contactless inductive coupling to convert electrical power in one section to mechanical power in another section where the sections are rotating and non-rotating sections of downhole oilfield tools, including the drilling assemblies containing rotating and non-rotating members. This direct transfer and conversion has the desirable characteristic of requiring fewer components than other tools that transfer electrical power to operate electrically controlled devices to perform mechanical functions such as operating pumps. Direct conversion means fewer parts, thus leading to more economical, reliable and compact tool designs.
In general, the present invention provides apparatus for power transfer over a nonconductive gap between rotating and non-rotating members of downhole oilfield tools. The gap may contain a non-conductive fluid, such as drilling fluid or oil for operating hydraulic devices in the downhole tool. The downhole tool, in one embodiment, is a drilling assembly wherein a drive shaft is rotated by a downhole motor to rotate the drill bit attached to the bottom end of the drive shaft. A substantially non-rotating sleeve around the drive shaft includes a plurality of independently operated force application members, wherein each such member is adapted to be moved radially between a retracted position and an extended position. The force application members are operated to exert the force required to maintain and/or alter the drilling direction. In the preferred system, one or more mechanically operated devices such as hydraulic units provide energy (power) to the force application members. A transfer device transfers electrical power between the rotating and non-rotating members, and the electric power is converted directly to mechanical power. An electronic control circuit or unit associated with the rotating member controls the transfer of power between the rotating member and the non-rotating member.
In a preferred embodiment, the present invention is particularly suited for a Rotary Closed-Loop System (RCLS) type tool for drilling deviated boreholes with very small hole sizes. A RCLS system is an automated directional drilling system that contains its own programmed controller and steering sub, and drills continuously in the rotary mode. A non-rotating, orienting sleeve controls steering expanding force application members. Precisely controlled force on the force application members produces resultant force vectors that maintain inclination alignment and direction within the program well path. Course corrections are made continuously while drilling, with no trips required for tool adjustments. Real-time surface monitoring permits changes to the wellpath program if desired. This technology increases the rate-of-penetration, improves hole quality, and enables greater extended reach capability. The embodiment may also comprise measurement while drilling (MWD), geosteering and automated rotary drilling capability.
In general, one or more steering ribs are controlled by hydraulic pressure. A motor located on the rotating shaft of a bottom hole assembly driving an axial piston pump in the non-rotating sleeve manages the generation of hydraulic pressure. The motor windings are positioned on the rotating shaft and a magnetically polarized rotor is located on the non-rotating sleeve. There would be one motor for controlling a hydraulic pump for each steering rib. Rotation control of the motor controls the variable piston pressure, and no electrical transmission to the sleeve is required to control the ribs. In the preferred embodiment, the motor will run in drilling mud. Feedback regarding the position of the non-rotating sleeve will be measured by sensors in the non-rotating sleeve or by markers. These methods of feedback and the sensors required are well known in the art. An added benefit of this arrangement is that no hydraulic pressure has to be transmitted from the rotating shaft to the sleeve.
In an alternative embodiment of the invention, a power transfer device transfers power from the non-rotating housing to the rotating drill shaft. The power transferred to the rotating drill shaft is directly converted to electrical power to operate one or more sensors or electrically operated devices in the drill bit and/or the bearing assembly.
The power transfer device may also be provided in a separate module above the mud motor to transfer power from a non-rotating section to the rotating member of the mud motor and the drill bit. The power transferred may be utilized to operate devices and sensors in the rotating sections of the drilling assembly, such as the drill shaft and the drill bit.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.